Perhaps it's a matter of semantics but I think there is a difference.
A train is a slang term for when two or more males has sex (usually) a female one after another. The sex is controlled by the guys and while the female is usually in a compromising position, generally bent over and facing away from the man. She passively accepts the sex acts as they are done to her. As one male finishes, the next male switches and continues having sex with the female. The typically continues until the males are done with her or in some cases, when the female says it’s over. A train can be three or substantially more individuals.
A train is one sided, the sex is more for the enjoyment of the males taking part rather than mutually among all parties. It’s also seen by many as very degrading act for the female. I suppose some women may not think it is degrading but to each his own.
A train is not to be confused with a gangbang or group sex. In a gangbang, the woman is engaging in sex with the men simultaneously and the enjoyment is more often mutual. From what I've seen and experienced, there can a blending the two for various reasons.
View attachment 7615238